An electrophotographic process is a process of obtaining a duplicate or copy by electrostatically charging an electrophotographic photoreceptor, image-exposing the photoreceptor to form electrostatic latent image, developing the latent image with a developer (toner), transferring the toner image formed on the photoreceptor onto a transfer paper, and fixing the transferred toner image.
The electrophotographic photoreceptor being used in the electrophotographic process is composed of an electrically conductive substrate having formed thereon a photo-sensitive layer as the fundamental structure, as the materials constituting the photosensitive layer, amorphous silicon (hydrogenated amorphous silicon) is known, and recently various improvements have been attempted.
The electrophotographic photoreceptor using the amorphous silicon (hereinafter, is referred to as amorphous silicon photoreceptor) is produced by forming an amorphous layer of silicon on an electrically conductive substrate by a glow discharging decomposition method of a silane (SiH.sub.4) gas and hydrogen atoms are contained in the amorphous silicon layer to show a good photoconductivity. The amorphous silicon photoreceptor has features that the surface hardness of the photosensitive layer thereof is high, the photoreceptor is excellent in abrasion resistance, has a high heat resistance, is excellent in electrical stability, has a wide spectral sensitivity, and has a high light sensitivity, and is an electrophotographic photoreceptor having an ideal property as an electrophotographic photoreceptor.
The amorphous silicon photoreceptor has the excellent characteristics as photoreceptor as described above but the dark resistance thereof is relatively low and hence has the disadvantage that the dark decay of the photoconductive layer is large and when the photoreceptor is charged, a sufficient charged potential is not obtained. In other words, the amorphous silicon photoreceptor has the disadvantage that in the case of charging the photoreceptor, image-exposing the photoreceptor to form an electrostatic latent image, and then developing the latent image, the surface potential on the photoreceptor is decayed before the image-exposure or the charges at the unexposed portions are decayed before the development step, whereby the charged potential necessary for development is hard to obtain.
The decay of the charged potential is liable to be changed by the influence of the surrounding conditions, and in particular, under a high-temperature high-humidity circumstance, the charged potential is greatly lowered. Furthermore, when the photoreceptor is repeatedly used, the charged potential is gradually lowered.
When a copy is made by using the electrophotographic photoreceptor showing such a large dark decay of the charged potential, the copy obtained has a low image density and a poor reproducibility of half tone.
For improving the aforesaid point, it has been proposed to form amorphous silicon as a photoconductive layer and applying amorphous silicon carbide, amorphous silicon nitride, or amorphous silicon oxide onto the photoconductive layer by a plasma CVD method to form a charge blocking layer which further functions, at the same time, as a surface protective layer.
However, in the amorphous silicon photoreceptor having the surface protective layer formed as described above, there is a problem that by repeating the copying operation, an image blurring occurs. This phenomenon is particularly severe at a high-humidity circumstance and hence such an amorphous silicon photoreceptor cannot be used for an ordinary electrophotographic process.
Also, a process of increasing the dark resistance and, at the same time, reducing the dielectric constant by doping carbon, nitrogen, oxygen, etc., into the amorphous silicon layer has been proposed as described in JP-A-56-62254 and JP-A-56-62255. (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, in the foregoing process, an optical gap is broadened, whereby the sensitivity in a long wavelength region is lost and thus the process cannot be used for a printer using a semiconductor laser.
Furthermore, a process of preventing the occurrence of interference fringes by most suitably selecting the reflective index and the thickness of the surface layer composed of amorphous silicon carbide, amorphous silicon nitride, etc., has been proposed as described in JP-A-61-29851. However, in the foregoing process, the surface layer is abraded by repeating the copying operation for a long period of time, whereby the surface layer loses the function of preventing the reflection of light. Also, the process cannot cope with the gap of the oscillation wavelength of a semiconductor laser by the change of the surrounding conditions.
Still further, amorphous silicon formed by a plasma CVD method has a high surface hardness but has a fault that the amorphous silicon is liable to be cracked and is weak to shock as compared to a selenium-series photosensitive layer and an organic photoreceptor. Accordingly, the amorphous silicon photoreceptor is scratched by a paper-releasing claw, etc., in a copying apparatus or a printer, which results in causing a problem that white points and black points are liable to form on the image of the copy.
Also, an amorphous silicon photoreceptor has many semispherical defects having a diameter of from 1 .mu.m to 30 .mu.m at the surface of the photosensitive layer and by repeating a copying operation, electrical or mechanical destructions occur at the defect portions of the layer to form white points and black points on the images formed, which reduces the quality of images.